The present invention relates generally to tunnel kilns employed in the process of baking carbonaceous products, and particularly to a method that impedes and prevents the deposition of coke on the walls of the ware space of such kilns without adverse effect on the carbonaceous products themselves.
As is well known in the art of baking carbon articles, such as electrodes used in electrolytic cells for making aluminum, plastic carbon aggregate and organic binder formulations comprise the green, unbaked carbon electrodes, the binder formulations including such aromatic elements as petroleum or coal tar pitch with or without the addition of commercial additives such as fillers and plasticizers. The organic binders that have been found satisfactory in the practice of baking carbon electrodes in tunnel kilns generate substantial quantities of volatile gaseous hydrocarbons and soot. These hydrocarbons and soot circulate in the ware space of the baking zone of the kiln and come in contact with the side walls of the baking zone. These walls, which can be porous, separate the baking zone from combustion chambers, located on opposed sides of the baking zone, such that these walls are quite hot, the temperature of the walls reaching a maximum of 1300.degree. C. When the gases contact the hot walls, the gases crack into hydrogen or lighter hydrocarbons and coke, with the coke depositing on the inwardly facing surfaces of the walls. These coke deposits are known as "the coke ring", and are of very hard variety resembling pyrolitic coke. Unless the deposits are removed, they will grow inwardly in the tunnel kiln and ware space and eventually obstruct and interfere with the passage of the carbon articles through the kiln. Thus, the operating life of the kiln is substantially reduced.
As is further well known, the operation of a tunnel kiln requires a pressure gradient in longitudinal flues of the kiln between the combustion chambers of the flues and the exhaust stacks located adjacent the beginning or entrance end of the kiln. The pressure gradient is necessary in order to remove products of combustion (i.e. carbon dioxide, water vapor, etc.) produced in combustion chambers of the flues, as the combustion chambers and flues dead-end at the downstream end of the baking zone. Heretofore, the gradient in the flues, and the resulting pressure differential between the ware space of the baking zone and the combustion chambers, have been controlled to prevent the burning of the products in the ware space of the baking zone. This has resulted in a pressure condition in the baking zone that not only does not limit or prevent the formation of the coke ring on the walls of the baking zone but, in addition, strings out the coke ring over a substantial distance on the walls of the furnace, the ring trailing off to zero deposition as the pressure differential reaches a positive value.
In U.S. Pat. No. 3,009,863 to Angevine, a variety of means is suggested for maintaining a positive pressure condition in the flues relative to the pressure in the portion of the kiln tunnel for baking carbon articles for the purpose of selectively burning the coke ring. Such means include the use of dampers or valve mechanisms in the exhaust lines from the flues to the exhaust stacks, adjustment of external sources of pressurizing atmosphere and variation of secondary air introduced to the flues adjacent the areas of coke deposit, incorporating restrictions or baffles within the flues at desired locations along their length, or by combinations of any two or more of such means. As an alternative to such means, the text of the patent suggests the use of intermittently operated steam jets for admitting steam to the baking area in amounts sufficient to support combustion of coke and soot.
Such means have only been partially successful in selectively burning the coke ring on the walls of the flues in the baking zone. This is true even with the use of tar collecting equipment associated with the lower temperature, preheating sections of the kiln. In the preheating sections, the binder in the electrodes is devolatilized into coke, which produces lower-boiling volatiles in the preheating area, and to a lesser degree in the high temperature baking zone. These volatiles include heavy tars and oils and fluoride salts which are collected by condensers and electrostatic precipitators for the purpose of removing such tars, oils and salts from the kiln gases. Such cleaning means have been troublesome to operate and allow accelerated growth of coke ring when not in operation. The sweep gas or main flow down the length of the tunnel is not capable of complete removal of the residual tars and oils and therefore, they are present in the kiln gases to crack and deposit on the walls of the baking zone. Hence, there is still the need for an effective, reliable means to control the ingress of combustion products into the baking zone for the purpose of impeding and preventing such deposits without burning the carbon articles in the baking zone.